feat(db): require explicit SQLite backend declarations

Signed-off-by: npub13n66s06epmqf2kc3v373ez8hj65cuzyvxzjf93vwpervxqn2u7jq2qd9je <8cf5a83f590ec0955b11647d1c88f796a98e088c30a492c58e0e46c3026ae7a4@buzz.block.builderlab.xyz>

Cargo.lock

@@ -1005,6 +1005,7 @@ name = "buzz-db"
 version = "0.1.0"
 dependencies = [
  "buzz-core",
+ "buzz-db-backend-macro",
  "chrono",
  "hex",
  "metrics",
@@ -1021,6 +1022,16 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "buzz-db-backend-macro"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+ "trybuild",
+]
+
 [[package]]
 name = "buzz-dev-mcp"
 version = "0.1.0"
@@ -3260,6 +3271,12 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "glob"
+version = "0.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e4eba85ea1d0a966a983acd07deee566e67395d2d96b6fb39e62b5a833f1eb0b"
+
 [[package]]
 name = "globset"
 version = "0.4.18"
@@ -9490,6 +9507,12 @@ dependencies = [
  "xattr",
 ]
 
+[[package]]
+name = "target-triple"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c3a6bfce3d99adfa72d24750a61f782f3036a81e7f86d8841ee1326deaebd171"
+
 [[package]]
 name = "tempfile"
 version = "3.27.0"
@@ -9503,6 +9526,15 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "termcolor"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06794f8f6c5c898b3275aebefa6b8a1cb24cd2c6c79397ab15774837a0bc5755"
+dependencies = [
+ "winapi-util",
+]
+
 [[package]]
 name = "termina"
 version = "0.3.3"
@@ -10180,6 +10212,21 @@ version = "0.2.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 
+[[package]]
+name = "trybuild"
+version = "1.0.120"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e605bf6b39357663d8ba4e984f8be8da8df6bb32e81031d6889024ea8fd68e4"
+dependencies = [
+ "glob",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "target-triple",
+ "termcolor",
+ "toml 1.1.2+spec-1.1.0",
+]
+
 [[package]]
 name = "tungstenite"
 version = "0.26.2"

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "crates/buzz-conformance",
     "crates/buzz-push-gateway",
     "crates/buzz-db",
+    "crates/buzz-db-backend-macro",
     "crates/buzz-pubsub",
     "crates/buzz-auth",
     "crates/buzz-search",

crates/buzz-db-backend-macro/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "buzz-db-backend-macro"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+publish = false
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1"
+quote = "1"
+syn = { version = "2", features = ["full"] }
+
+[dev-dependencies]
+trybuild = "1"

crates/buzz-db-backend-macro/src/lib.rs

@@ -0,0 +1,104 @@
+//! Compile-time enforcement for explicit SQLite decisions on public `Db` methods.
+//!
+//! The impl-level attribute inspects only public associated methods. Private and
+//! `pub(crate)` plumbing is intentionally outside this policy boundary.
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, Attribute, ImplItem, ItemImpl, LitStr, Visibility};
+
+#[proc_macro_attribute]
+pub fn enforce_sqlite_backend_declarations(_args: TokenStream, input: TokenStream) -> TokenStream {
+    let mut item = parse_macro_input!(input as ItemImpl);
+    let mut errors = Vec::new();
+    let mut inventory = Vec::new();
+
+    for impl_item in &mut item.items {
+        let ImplItem::Fn(method) = impl_item else {
+            continue;
+        };
+        if !matches!(method.vis, Visibility::Public(_)) {
+            continue;
+        }
+
+        let mut markers = Vec::new();
+        let mut retained = Vec::new();
+        for attr in method.attrs.drain(..) {
+            if attr.path().is_ident("sqlite_backend") {
+                markers.push(attr);
+            } else {
+                retained.push(attr);
+            }
+        }
+        method.attrs = retained;
+
+        let name = method.sig.ident.to_string();
+        match markers.as_slice() {
+            [] => errors.push(syn::Error::new(
+                method.sig.ident.span(),
+                format!("public Db method `{name}` is missing #[sqlite_backend(...)]"),
+            )),
+            [marker] => match parse_marker(marker) {
+                Ok(reason) => inventory.push((name, reason)),
+                Err(error) => errors.push(error),
+            },
+            _ => errors.push(syn::Error::new(
+                method.sig.ident.span(),
+                format!("public Db method `{name}` has duplicate #[sqlite_backend] markers"),
+            )),
+        }
+    }
+
+    inventory.sort_by(|a, b| a.0.cmp(&b.0));
+    let names = inventory
+        .iter()
+        .map(|(name, _)| LitStr::new(name, proc_macro2::Span::call_site()));
+    let reasons = inventory.iter().map(|(_, reason)| match reason {
+        Some(reason) => quote! { Some(#reason) },
+        None => quote! { None },
+    });
+    let errors = errors.into_iter().map(|e| e.to_compile_error());
+
+    quote! {
+        #(#errors)*
+        #item
+        #[doc(hidden)]
+        pub const SQLITE_BACKEND_INVENTORY: &[(&str, Option<&str>)] = &[#((#names, #reasons)),*];
+    }
+    .into()
+}
+
+fn parse_marker(attr: &Attribute) -> syn::Result<Option<LitStr>> {
+    let mut result = None;
+    attr.parse_nested_meta(|meta| {
+        if meta.path.is_ident("implemented") {
+            if result.is_some() {
+                return Err(meta.error("duplicate backend decision"));
+            }
+            result = Some(None);
+            return Ok(());
+        }
+        if meta.path.is_ident("unsupported") {
+            if result.is_some() {
+                return Err(meta.error("duplicate backend decision"));
+            }
+            let value = meta.value()?.parse::<LitStr>()?;
+            if value.value().trim().is_empty() {
+                return Err(syn::Error::new(
+                    value.span(),
+                    "unsupported reason must not be empty",
+                ));
+            }
+            result = Some(Some(value));
+            return Ok(());
+        }
+        Err(meta.error("expected `implemented` or `unsupported = \"reason\"`"))
+    })?;
+    result.ok_or_else(|| {
+        syn::Error::new(
+            attr.span(),
+            "backend decision must be `implemented` or `unsupported = \"reason\"`",
+        )
+    })
+}

crates/buzz-db-backend-macro/tests/compile.rs

@@ -0,0 +1,9 @@
+#[test]
+fn ui() {
+    let tests = trybuild::TestCases::new();
+    tests.compile_fail("tests/ui/missing_marker.rs");
+    tests.compile_fail("tests/ui/duplicate_marker.rs");
+    tests.compile_fail("tests/ui/empty_reason.rs");
+    tests.compile_fail("tests/ui/malformed_marker.rs");
+    tests.pass("tests/ui/valid_markers.rs");
+}

crates/buzz-db-backend-macro/tests/ui/duplicate_marker.rs

@@ -0,0 +1,9 @@
+use buzz_db_backend_macro::enforce_sqlite_backend_declarations;
+struct Db;
+#[enforce_sqlite_backend_declarations]
+impl Db {
+    #[sqlite_backend(implemented)]
+    #[sqlite_backend(implemented)]
+    pub fn duplicated(&self) {}
+}
+fn main() {}

crates/buzz-db-backend-macro/tests/ui/duplicate_marker.stderr

@@ -0,0 +1,5 @@
+error: public Db method `duplicated` has duplicate #[sqlite_backend] markers
+ --> tests/ui/duplicate_marker.rs:7:12
+  |
+7 |     pub fn duplicated(&self) {}
+  |            ^^^^^^^^^^

crates/buzz-db-backend-macro/tests/ui/empty_reason.rs

@@ -0,0 +1,8 @@
+use buzz_db_backend_macro::enforce_sqlite_backend_declarations;
+struct Db;
+#[enforce_sqlite_backend_declarations]
+impl Db {
+    #[sqlite_backend(unsupported = "  ")]
+    pub fn empty(&self) {}
+}
+fn main() {}

crates/buzz-db-backend-macro/tests/ui/empty_reason.stderr

@@ -0,0 +1,5 @@
+error: unsupported reason must not be empty
+ --> tests/ui/empty_reason.rs:5:36
+  |
+5 |     #[sqlite_backend(unsupported = "  ")]
+  |                                    ^^^^

crates/buzz-db-backend-macro/tests/ui/malformed_marker.rs

@@ -0,0 +1,8 @@
+use buzz_db_backend_macro::enforce_sqlite_backend_declarations;
+struct Db;
+#[enforce_sqlite_backend_declarations]
+impl Db {
+    #[sqlite_backend(sometimes)]
+    pub fn malformed(&self) {}
+}
+fn main() {}

crates/buzz-db-backend-macro/tests/ui/malformed_marker.stderr

@@ -0,0 +1,5 @@
+error: expected `implemented` or `unsupported = "reason"`
+ --> tests/ui/malformed_marker.rs:5:22
+  |
+5 |     #[sqlite_backend(sometimes)]
+  |                      ^^^^^^^^^

crates/buzz-db-backend-macro/tests/ui/missing_marker.rs

@@ -0,0 +1,10 @@
+use buzz_db_backend_macro::enforce_sqlite_backend_declarations;
+
+struct Db;
+
+#[enforce_sqlite_backend_declarations]
+impl Db {
+    pub fn omitted(&self) {}
+}
+
+fn main() {}

crates/buzz-db-backend-macro/tests/ui/missing_marker.stderr

@@ -0,0 +1,5 @@
+error: public Db method `omitted` is missing #[sqlite_backend(...)]
+ --> tests/ui/missing_marker.rs:7:12
+  |
+7 |     pub fn omitted(&self) {}
+  |            ^^^^^^^

crates/buzz-db-backend-macro/tests/ui/valid_markers.rs

@@ -0,0 +1,18 @@
+use buzz_db_backend_macro::enforce_sqlite_backend_declarations;
+
+struct Db;
+
+#[enforce_sqlite_backend_declarations]
+impl Db {
+    #[sqlite_backend(implemented)]
+    #[allow(dead_code)]
+    pub fn implemented(&self) {}
+
+    /// Deliberately unsupported operation.
+    #[sqlite_backend(unsupported = "requires postgres-only advisory locks")]
+    pub async fn unsupported(&self) {}
+
+    fn private(&self) {}
+}
+
+fn main() {}

crates/buzz-db/Cargo.toml

@@ -9,6 +9,7 @@ description = "Postgres event store and data access layer for Buzz"
 
 [dependencies]
 buzz-core = { workspace = true }
+buzz-db-backend-macro = { path = "../buzz-db-backend-macro" }
 sqlx        = { workspace = true }
 tokio       = { workspace = true }
 serde       = { workspace = true }

crates/buzz-db/SQLITE_BACKEND_INVENTORY.md

@@ -0,0 +1,15 @@
+# SQLite backend inventory
+
+Generated from `#[sqlite_backend(...)]` declarations in `crates/buzz-db/src/lib.rs`.
+
+> ⚠️ **PROVISIONAL — classification in progress.** Most declarations are mechanical placeholders and this table is not yet valid S6 input.
+
+Regenerate after changing declarations with:
+
+```sh
+cargo test -p buzz-db backend_inventory_is_current -- --nocapture
+```
+
+| Method | SQLite status | Reason |
+| --- | --- | --- |
+| `usage_community_count` | unsupported | usage analytics is PostgreSQL-only |